Display devices, such as liquid crystal displays (LCD) and cathode ray tubes (CRT), have been widely used to display various images represented by digital data. In this regard, analog data which has been converted into digital information, or information which was originally generated as digital information, can be readily displayed on an LCD or a CRT display by providing a horizontal synchronization signal at the beginning of each row of pixel information, and subsequently providing pixel information for each pixel in a row. Following the last pixel for a given row, a new horizontal synchronization signal is initiated to begin sending new pixel information to the next row. This pattern is repeated until the entire display has been created, and then the process begins again in the next frame in the process of generating a display image.
In order to produce a sharp image on the display, it is imperative that the horizontal synchronization signal for each row is properly synchronized with the horizontal synchronization signal for that row. Should the first portion of pixel information not coincide precisely with the horizontal synchronization signal, the subsequent portions of pixel information cannot be supplied to the pixel elements in the proper manner prior to the next row's horizontal synchronization signal being received. As a result, the pixel information becomes shifted in time, causing unwanted distortions in the displayed image. In the situation where the first portion of pixel information does coincide precisely with the horizontal synchronization signal, but the number of portions of pixel information which are supplied to the display do not correspond to the number of pixel elements in the row, the display is again distorted due to the subsequent horizontal sync signal failing to correspond with the beginning of the following row.
In an attempt to overcome the problem of image distortion due to either the horizontal synchronization signal failing to correspond with the first portion of pixel information or the number of portions of pixel information not corresponding with the number of pixel elements in a row, a phase locked loop (PLL) has been employed to enable a pixel clock to be precisely matched in frequency and phase with the horizontal synchronization signal. In this manner, the first portion of pixel information coincides with the horizontal synchronization signal, and the total number of portions of pixel information supplied are appropriate for the number of pixel elements in a given row. In order to match the frequency and phase of the pixel clock and the horizontal synchronization signal, two gain values are required to be set for the PLL. The first gain value that is required to be set is the phase detector gain (PDG), while the other gain which is required to be set is the voltage controlled oscillator gain (VCO). The total closed loop gain of the PLL is simply the PDG multiplied by the VCO gain.
The correct closed loop gain for synchronizing properly the pixel clock with the horizontal synchronization signal depends upon manufacturers, including the input/output frequency ratio, the loop filter parameters, and also upon environmental noise. In fact, there are so many different factors that can adversely affect the operation of the PLL circuit, that two PLL circuits of the same design may have slightly different gain values for a similar graphics mode to enable the pixel clock to be synchronized with the horizontal synchronization signal.
In the past, a specific closed loop gain value was passively preset for each graphics mode. However, the initial preset closed loop gain value frequently, if not always, become inadequate for proper synchronization over time. As a result, it becomes necessary for the user to adjust the gain value as required. Due to the numerous possible combinations of gain values, the adjustment of the entire closed loop gain results in confusion and inconvenience for the user.
Therefore, it would be highly desirable to have a new and improved display device image stabilization apparatus, which helps maintain the desired synchronization of pixel information driving a display unit for various graphic modes. Such a new and improved system should overcome the problems associated with the inconvenience of requiring the user to make repeated and unwanted adjustments to maintain the desired stability of the system. Such a display device image stabilization arrangement should operate automatically, or at least require little intervention by the user.